Biodiversity of freshwater fishes of Trinidad and Tobago, West Indies

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Abstract

The proximity of the speciose South American continent to
Trinidad and Tobago ensures that these islands have a rich and
dynamic fauna. According to the equilibrium theory of island
biogeography (MacArthur & Wilson 1967), these islands should
be subjected to frequent immigrations from the nearby
continent, and these additions will, in response, fuel local
extinctions to maintain a balance of species richness. The
aquatic habitat is further impacted by man's activities,
(Government of Trinidad and Tobago 1962; Government of
Trinidad and Tobago 1976b) which have the potential to amplify
the natural rates of immigration and extinction. Despite the
possible consequences, the effects of this disturbance on the
fish fauna have not been studied. A survey of the islands'
freshwater fishes was therefore carried out to investigate the
natural spatial and temporal dynamics of local fish
communities, and the effects of anthropogenic perturbations,
on these. These baseline data can be used as a benchmark to
address some of the problems that are threatening the fish
diversity of Trinidad and Tobago.
The fish fauna of Trinidad and Tobago is diverse. Total
species richness was estimated at between 37 and 40.
Four
zoogeographic zones were recognised. A zone of Antillean
fishes included Tobago and the north coast of Trinidad. A
zone of recent colonising South American fauna existed along
the south coast of Trinidad. The rest of Trinidad contained a
relict South American fauna dating back to the time when
Trinidad was still part of the mainland. Included here was a
centrally located zone of unstable fauna at risk of local
extinction.
The fish fauna was temporally dynamic due to frequent
colonisation and extinction events. Examination of the
historic record showed that observed species richness varied
from 38 to 43 between the mid 1950s and the present. During
this time there were 15 introductions and 12 local
extinctions. A conservative estimate was that a new species
was recorded for Trinidad almost every three years. The
geographic distribution of species also showed temporal
changes which indicated a natural tendency of the fauna to
vary over time.
Human interference, particularly the introduction of exotic
fish species and long-term habitat alteration, has affected
species diversity. Forty seven percent of the new
introductions and 43% of the extinctions were human-introduced
exotics. About four fifths of sites in Trinidad, and almost
one fifth of the sites in Tobago, were either perturbed or
polluted. Polluted rivers coincided with areas of high
urbanisation and industrial development in the west and
southwest of Trinidad.
Several effects of human interference on the fauna were
recorded. Almost 8% of the sites examined contained one of
the three exotic species still extant on the island. At each
of these sites, the exotic species accounted for between 1.3%
and 80.4%, by number, of the fish caught. Some of the effects
of habitat disturbance on individual sites were increased
frequency of diseases, extirpation of species, changes in
species richness and other diversity measures, and the
eventual regression of the fish community to opportunistic
species (r-strategists).
The potential of two fishes, Poecilia reticulata and Astyanax
bimaculatus, as indicator species was examined. Astyanax
showed better potential as an indicator of habitat quality as
it was not found in depauperate communities, typical of
severely disturbed habitats, and its proportional abundance
and biomass were negatively affected by pollution. Poecilia
populations, on the other hand, were found to be insensitive
to habitat quality when the above-mentioned criteria were
used. They did, however, have a high frequency of diseased
individuals at polluted sites.
One of the aims of conservation is to protect that portion of
biodiversity most at risk of extinction, the rare species
(Rabinowitz 1986). Over 70% of freshwater fish species found
in Trinidad and Tobago were classified as rare in these
islands. This fact, in addition to the loss of diversity
recorded for some sites indicates that the implementation of a
management strategy for the conservation of the freshwater
fish fauna of Trinidad and Tobago is imperative. The
management strategy should focus on the amelioration and
protection of aquatic habitats since at least 80% of the rare
species had either a restricted geographic distribution or
narrow habitat specificity. Additionally, protection from
overexploitation should be offered to commercially important
species with only small populations. Finally, a minimum
sample size of 35 sites, spread over different zoogeographic
areas, is recommended for estimating species richness for
monitoring, an intrinsic part of any management strategy.

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